Cryogenic pumps function by condensing and adsorbing gases on to very cold surfaces. The surfaces are typically cooled using a Helium gas refrigerator. A cryopump acts essentially as a gas freezer and storage device. Once the gas molecules are frozen onto the surface, they no longer exert pressure in the evacuated chamber. Once the surfaces are full or saturated, they must be regenerated to restore pumping performance.
Typically, cryopumps operate as two stage pumps. The first stage operates at a temperature around 80 degrees K with the intent to condense water vapor and carbon dioxide. The second stage operates at a lower temperature, typically 20 degrees K in order to condense other common gases such as oxygen, nitrogen, and argon. The second stage also normally includes an activated charcoal charge which acts to adsorb Helium and Hydrogen.
The mechanism by which gas molecules arrive at the pumping surface and are condensed or adsorbed is complicated. The gas molecule itself has many interactions with the pumping surface. First, the gas molecule must attach itself to the surface in a process called adsorption. Adsorption can only occur after the molecule has lost sufficient energy that it can be held on the surface by weak forces called Van Der Waals forces. These forces are very weak and a molecule must attain a low energy state for this to occur. One way for molecules to lose energy is through collisions. Inelastic collisions are those where molecules lose energy by transferring it to other molecules when they hit. A gas molecule that enters the pump may collide with the pumping surface and simply bounce off. A gas molecule may bounce many times before it has lost sufficient energy to adsorb to the pumping surface. Each time that a molecule bounces, there is a chance that it will be directed back out of the pump into the evacuated space. Because cryopumping depends upon the random motion of gases, keeping gas molecules inside of the pump is of prime interest.
Whether or not the molecule bounces depends on many factors. These include the atom's or molecule's energy level, its kinetic energy, angle of incidence, surface profile, and surface temperature.
A typical two-step cryogenic pump 11 is shown in FIG. 1. The pump includes an outer vacuum vessel 12 having a cylindrical opening at one end circumscribed by a mounting flange 13. Flange 13 includes bores 14 for securance of the same in accordance with standard practice via a gate valve or the like to a vacuum chamber to be pumped. Vessel 12 houses a conventional refrigeration cylinder 16 axially within the same. Such cylinder supports and provides the desired low temperatures to the first and second stages of the pump. In this connection, in accordance with conventional practice the cylinder 16 relies on the condensation of helium to obtain the low temperatures. A compressor (not shown) supplies room temperature helium under pressure to the pump via connection 17, where the helium expands to cool the two stages of the pump.
The first, or initial stage of the pump is made up of a radiation shield 18 which supports an array 21 of coaxial annular fins. The array 21 provides the dual function of acting not only to provide an extended surface area for condensation at the initial stage, but also to protect the lower temperature condensation and adsorbent second stage from direct line-of-sight exposure to the gases to be pumped. With respect to the former, such array is constructed to provide overlapping surface areas which block such line-of-sight but yet permit passage therethrough of those gases which do not condense on its surface areas. It is thermally coupled via the radiation shield 18 to the central area of the cylinder 16 so as to be maintained at a temperature within the range of 50.degree.-80.degree. K. A washer 22 made of a good thermally conductive material such as indium is provided at the physical connection of the shield to the cylinder 16 to assure good thermal conduction between the two.
The second stage of the pump includes an array of condensing panels or plates 23. In the prior art a portion of these panels is generally covered with a coating of adsorbent material such as activated charcoal. The coating is applied to the surface with an adhesive. The adhesive is applied in such a manner that the adsorbent particles are exposed. In U.S. Pat. No. 5,450,729, the coating material is applied with an adhesive which is transparent to passage of the gases which are being pumped.
The second stage usually operates at a temperature between 10-20 degrees K. On to these panels gases such as nitrogen, oxygen, and argon adhere. Gases that are not condensed onto a second stage panel can lastly be adsorbed into the activated charcoal adhering to the second stage array as described above.
In order to pump and hold more gases, the pumping surface area can be increased. The common method of accomplishing this is to place more baffle plates into the pump. This increases the chance that gases can attach and condense. Doing this reduces the amount of open area between the baffle plates. This increased restriction makes it more difficult for gases to get to the other baffle plates in the array. The result is slower pumping speed. Cryopumping is a slow process to begin with, so any loss of pumping speed is undesirable.
Since a cryopump is essentially a storage device it must be periodically emptied. This is called regeneration. The process of regeneration involves shutting down the pump, warming it up, and circulating gas through it. It is much the same as defrosting a refrigerator. The amount of gas a cryopump can freeze depends upon the temperature and surface area of the arrays. As the pumping surfaces continue to accumulate frozen material, a temperature gradient begins to develop across the frozen layer. When there is sufficient thickness of material, the temperature gradient becomes so great that the pumping surface can no longer accumulate more gases. At this point, the pump stalls. An equilibrium is set up where gases are freezing as fast as they sublimate. Due to this equilibrium, no additional gases can be pumped. Before the point of stalling is reached, the pump performance steadily degrades. The only way to cope with this loss of performance is to regenerate the pump. Each manufacturer has specifications for the total recommended gas load that a pump can accommodate. Once this load has been reached, the pump must be regenerated. As the pump is being regenerated, the equipment that it is servicing cannot be used. If the capacity of the pump could be increased, less frequent regeneration would be required.